1. Field of the Invention
The present invention relates generally to a fiber optic radiation sensor (dose rate meter) and dosimeter and more specifically, to a fiber optic radiation dosimeter for monitoring radiation sources such as ultraviolet, x-ray, gamma radiation, beta radiation, and protons.
2. Description of the Background Art
Thermoluminescent (TL) phosphors have been used for many years to monitor radiation exposure levels. These dosimeters measure the accumulated radiation exposure over a period of time, ranging from minutes, to days to years. Phosphor materials such as metal-ion-activated lithium fluoride (LiF), or calcium fluoride (CaF.sub.2) are commonly used in "TLD badges" to monitor personnel exposure to radiation. These dosimeters are generally prepared from powders of the phosphor that are pressed into opaque white pellets. When exposed to ionizing radiation, such as deep ultraviolet, x-ray or gamma radiation, free electrons are generated and are trapped in the material. The electrons remain trapped until a source of heat is applied to the material to stimulate the release of the electrons. The electrons recombine at a luminescence center in the material resulting in the emission of light. The amount of light emitted is proportional to the amount of radiation exposure. TLD phosphors have a number of problems that must be overcome to be used with confidence. TLD phosphors are limited in size because the opacity of the pellet limits the signal to light generated near the surface. As a result, the dimensions of commercial TLD dosimeters are limited, thereby limiting the dynamic range and overall sensitivity of the dosimeter. Although TLD phosphors must be heated in order to function, heating is also the origin of the most significant problems with TLD dosimeters. Heating irreversibly erases the stored information in the dosimeter. Thermoquenching of the signal at elevated temperature reduces the sensitivity. Finally, the sensitivity of the dosimeter changes upon heating such that the sensitivity must be reset before reuse.
Optically stimulated luminescent (OSL) phosphors operate much the same way as TL phosphors except the recombination luminescence is stimulated optically rather than thermally. Thus, OSL dosimeters avoid all of the problems caused by heating in TLD dosimeters. OSL readout of an OSL phosphor typically need not erase all of the stored information, providing the opportunity to perform subsequent OSL or TL readouts of the dose. The sensitivity of OSL dosimeters is not reduced by thermoquenching and it is not changed by the readout since the OSL dosimeter is not heated. Powdered OSL phosphors, however, are still opaque and experience the drawbacks associated with poor optical quality just as in the case of TLD phosphors.
Glasses have been considered previously as potential TLD phosphors since it was recognized that the optical transparency of glass offers the advantage of more efficient light collection. The effectiveness of these glasses for TLD applications has been limited for a number of reasons, including low readout temperatures, low sensitivity compared to crystalline phosphors and low saturation doses. To some extent, these problems were overcome by use of the glasses described in U.S. Pat. No. 5,656,815 to Huston et al, the entirety of which is incorporated by reference herein for all purposes. The glasses described in that patent are highly favorable for TLD dosimetry. U.S. Pat. No. 5,811,822 to Huston et al, issued Sep. 22, 1998 and entitled "OPTICALLY TRANSPARENT, OPTICALLY STIMULABLE GLASS COMPOSITES FOR RADIATION DOSIMETRY" (the entirety of which is incorporated by reference herein for all purposes) describes novel glass phosphor materials that exhibit highly favorable characteristics for OSL dosimetry applications.
Fiber optic coupled remote dosimeters using TLD and OSL phosphors have also been described. One system, described in U.S. Pat. No. 4,999,504, issued Mar. 12, 1991 to Braunlich et al., utilizes powdered TL phosphors attached to the end of a 0.6 mm diameter optical fiber. An absorbing material is applied to one surface of the phosphor and a diode laser is used to heat the absorber which in turn heats the TL material by diffusive heating. This system is described as a remote fiber optic laser TLD system. The performance of the system is limited in several ways. First, the TL material must be very thin, approximately 0.1 mm, to allow the laser heating source to be transmitted through the TL material to the absorber material. As a consequence, in order to attain sufficient TL sensitivity, the diameter of the TL material and the fiber must be fairly large. A similar approach has been described for a fiber optic coupled OSL dosimeter (U.S. Pat. No. 5,030,834, issued Jul. 9,1991, to Lindmayer et al.). In this case, the OSL phosphor powder is attached to the end of a commercial fiber using an epoxy binder. Because of the high degree of scattering in the phosphor powder, only a very thin layer of powder can be used, thereby seriously limiting the sensitivity. U.S. Pat. No. 5,606,163 to Huston et al., the entirety of which is incorporated by reference herein for all purposes, discloses a fiber-optic coupled remote dosimeter that uses a novel laser heated glass fiber dosimeter to accurately measure radiation exposure for doses from .about.1 rad to .about.8000 rad.